1. Field of the Invention
The invention relates in general to an imaging device, and more particularly to an imaging device for sensing skin textures.
2. Related Art
In the technology of sensing the capacitor of the skin of the human body, a fingerprint sensor for sensing the texture of the finger or a touch panel or a touch screen for capacitor touch control may be applied.
More particularly, the sensor for sensing the texture of the skin has a portion for contacting the texture of the skin, and the basic structure of the portion includes an array-type sensing member. That is, several sensing members having the same structure constitute a two-dimensional sensor array. For example, when the finger having ridges and valleys is placed thereon, the ridge is in direct contact with the sensor, and the valley is separated from the sensor by a gap. A two-dimensional capacitance image of the texture of the finger may be acquired according to the distances between the sensing members and the ridges and valleys, and this is the basic principle of the capacitive skin texture sensor.
In the most-frequently seen sensing member structure, the skin in contact with the sensor may be regarded as an electrode plate with the same potential due to the conductive property of the human body. Each sensing member is a plate electrode, and a capacitor may be formed between each sensing member and the skin. A dead skin layer of the skin of the finger and a sensor protection layer in direct contact with the dead skin layer are located between the two electrode plates. The protection layer may include a single insulating layer or multiple insulating layers and must have the properties of withstanding the environment erosion, the force impact and the electrostatic discharge.
In order to achieve the above-identified properties of the protection layer, the most direct method is to increase the thickness of the protection layer so that the property of the high intensity and the ability of withstanding the electrostatic discharge may be obtained. However, the too-thick protection layer causes the very small sense capacitance (C_cell) and thus decreases the sensing sensitivity.